* '''A''' - In early phases of development the posterior circulation relies almost entirely from blood supply coming from the anterior circulation through carotid-vertebrobasilar anastomoses.

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* '''B and C''' - As the posterior fossa structures and the occipital lobe grow, the posterior circulation becomes progressively independent from the anterior circulation with obliteration of the anterior-posterior anastomoses from caudal to rostral maintaining in the majority of adult only one connection between the distal basilar arteries with the carotid artery via the posterior communicating artery.

Introduction

This page relates specifically to vascular development associated with the central nervous system (CNS). There is an additional page that looks at vascular development and remodelling. The CNS is extensively vascularised as neurons have a mainly oxygen requiring (aerobic) metabolism. At the cellular level the finer blood vessel endothelium is separated from direct contact by glial cells, that form the so-called "blood-brain barrier".[1]

There is also an extensive vascular network throughout the surrounding meninges, the skull bone, and the scalp. During the embryonic period blood vessels are also modified to form the extensive choroid plexus that lies within the ventricular space and is a source of the cerebrospinal fluid (CSF).

Some Recent Findings

Formation of the circle of Willis during human embryonic development[2] "The circle of Willis (CW) is a circulatory anastomosis that supplies blood to the brain and adjacent structures. We examined the timing of formation of CW in 20 Japanese human embryo samples by using 3-dimensional reconstruction of serial histological sections. The CW was closed in 1 (n = 6), 2 (n = 8), 2 (n = 3) and 2 (n = 3) samples at Carnegie stages 20, 21, 22, and 23, respectively. The CW was unclosed in 13 samples (unclosed at ACOM alone, 6 samples; ACOM and bilateral P1, 4; left PCOM and right P1, 1; right PCOM and right P1, 1; ACOM and left PCOM, 1). It was difficult to predict whether the circle would close during further development, as such variations frequently exist in adults."

Foxc1 is required for early stage telencephalic vascular development[3] "The brain vascular system arises from the perineural vascular plexus (PNVP) which sprouts radially into the neuroepithelium and subsequently branches off laterally to form a secondary plexus in the subventricular zone (SVZ), the subventricular vascular plexus (SVP). The process of SVP formation remains to be fully elucidated. We investigated the role of Foxc1 in early stage vascular formation in the ventral telencephalon. Results: The Foxc1 loss of function mutant mouse, Foxc1ch/ch , showed enlarged telencephalon and hemorrhaging in the ventral telencephalon by E11.0. The mutant demonstrated blood vessel dilation and aggregation of endothelial cells in the SVZ after the invasion of endothelial cells through the radial path, which lead to failure of SVP formation. During this early stage of vascular development, Foxc1 was expressed in endothelial cells and pericytes, as well as in cranial mesenchyme surrounding the neural tube."

Review - The human brain intracerebral microvascular system: development and structure.[4] "The capillary from the meningeal inner pial lamella play a crucial role in the development and structural organization of the cerebral cortex extrinsic and intrinsic microvascular compartments. Only pial capillaries are capable of perforating through the cortex external glial limiting membrane (EGLM) to enter into the nervous tissue, although incapable of perforating the membrane to exit the brain. Circulatory dynamics and functional demands determine which capillaries become arterial and which capillaries become venous."

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(35 days) development of the middle cerebral artery (MCA) is first identified as small buds originating proximal to the anterior cerebral artery (ACA) on the anterior division of the primitive internal carotid artery (ICA).

Early development - the posterior circulation relies almost entirely from blood supply coming from the anterior circulation through carotid-vertebrobasilar anastomoses.

Later development - as the posterior fossa structures and the occipital lobe grow, the posterior circulation becomes progressively independent from the anterior circulation with obliteration of the anterior-posterior anastomoses from caudal to rostral maintaining in the majority of adult only one connection between the distal basilar arteries with the carotid artery via the posterior communicating artery.

Blood-Brain Barrier

Fig 11. Endothelial tight junctions Regions of overlap between neighboring endothelial cells, illustrating range of variation in structure of the intercellular cleft. In Fig. 11, the cleft is obliterated by a tight junction (arrow) extending throughout most of its length. The region of the intercellular cleft indicated by the arrow in Fig. 11 is shown in inset at higher magnification to illustrate that the total width of the tight junc- tion (between arrows at right) is less than twice the width of the adjacent plasma membrane (between arrows at left). Normal mouse; uranyl acetate block stain. Fig. 11, X 170,000. Fig. 11 inset, X 420,000.

Fig 12. Astrocytic end feet (A) lying between a myelinated axon and the basement lamina of the vascular endothelium. A cleft between the astrocytic end feet extends from the basement lamina to a perivascular myelinated axon and appears open except near the blood vessel where it is invaded by some basement lamina material. Normal mouse; uranyl acetate block stain. X 150,000.

pericytes - (Rouget cells) cells located at the abluminal surface of microvessels close to endothelial cells, mainly found associated with CNS vessels and involved in vessel formation, remodeling and stabilization.

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding. (More? Embryology History | Historic Embryology Papers)

diencephalon - the caudal portion of forebrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). (cavity- 3rd ventricle) Forms the thalmus and other nuclei in the adult brain. (sc-My-Met-Mes-Di-Tel)

dorsal root ganglia - (spinal ganglia) sensory ganglia derived from the neural crest lying laterally paired and dorsally to the spinal cord (in the embryo found ventral to the spinal cord). Connects centrally with the dorsal horn of the spinal cord.

dura mater- "tough" (Latin, mater = mother) used in reference to the tough outer layer of the brain meninges.

efferent - refers to the direction of conduction from the central nervous system toward the periphery. Afferent is in the opposite direction.

ependyma - epithelia of remnant cells after neurons and glia have been generated and left the ventricular zone.

floorplate - early forming thin region of neural tube closest to the notochord.

ganglia - (pl. of ganglion) specialized neural cluster within either the CNS or PNS.

glia - supporting, non-neuronal cells of the nervous system. Generated from the same neuroepithelial stem cells that form neurons in ventricular zone of neural tube. Form astrocytes, oligodendrocytes.

grey matter - neural regions containing cell bodies (somas) of neurons. In the brain it is the outer layer, in the spinal cord it is inner layer. (see white matter white matter).

growth factor - usually a protein or peptide that will bind a cell membrane receptor and then activates an intracellular signaling pathway. The function of the pathway will be to alter the cell directly or indirectly by changing gene expression. (eg SHH).

hydrocephalus - abnormality as the result of an imbalance between the rate at which the CSF is being formed and the rate at which the CSF is passing through the arachnoidal villi back into the blood (hydrocephalus rate is a function of the degree of imbalance in these two). Very small imbalance exhibit subtle, if any, symptoms. Large imbalances will have rapidly evolving symptoms of unmistakable import.

mantle layer - layer of cells generated by first neuroblasts migrating from the ventricular zone of the neural tube. Layers are rearranged during development of the brain and spinal cord. (Ven-Man-Mar-CP)

neuromere - (prosomere) the model units for segmental brain development regions based upon a series of neural tube transverse subunits.

neuron - The cellur "unit" of the nervous system, transmitting signals between neurons and other cells. The post-mitotic cells generated from neuroepithelial stem cells (neuroblasts) in ventricular zone of neural tube.

neuropore - opening at either end of neural tube cranial (rostral, anterior) neuropore closes (day 25) about 2 days before caudal (posterior) that closes at somite level 32 to 34. Neural Tube Defects (NTDs) can be due to failure of these two neuropores to close.

optic nerve - (cranial nerve II, CN II) retinal ganglion neurons project from the retina as a tract into the brain (at the level of the diencephalon) associated with vision.

optic vesicle - diencephalon region of neural tube outgrowth that forms the primordia of the retina associated with vision.

otocyst - (otic vesicle) sensory placode that sinks into mesoderm to form spherical vesicle (stage 13/14 embryo) that will form components of the inner ear associated with hearing.

pars - (L. part of)

pharyngeal arch - (branchial arch, Gk. gill) form the main structures of the head and neck. Humans have 5 arches appearing in week 4 that form 4 external swellings, each arch has a pouch, membrane and cleft.

pharynx - uppermost end of GIT, beginning at the buccopharyngeal membrane and at the level of the pharyngeal arches.

prosomere - (neuromere) a model for segmental brain development based upon a series of neural tube transverse subunits. PMID 12948657

Rathke's pouch - a portion of the roof of the pharynx pushes upward towards the floor of the brain forming the anterior pituitary (adenohypophysis, pars distalis, pars tuberalis pars intermedia). Where it meets a portion of the brain pushing downward forming the posterior pituitary (neurohypophysis, pars nervosa). Rathke's pouch eventually looses its connection with the pharynx.

sonic hedgehog - (shh) secreted growth factor that binds patched (ptc) receptor on cell membrane. SHH function is different for different tissues in the embryo. In the nervous system, it is secreted by the notochord, ventralizes the neural tube, inducing the floor plate and motor neurons.

telencephalon - the cranial portion of forebrain after it divides into 2 parts in the 5 secondary vesicle brain (week 5). (cavity- lateral ventricles and some of 3rd ventricle) Forms the cerebral hemispheres in the adult brain. (sc-My-Met-Mes-Di-Tel)

ventricles - the fluid-filled interconnected cavity system with the brain. Fluid (cerebrospinal fluid, CSF) is generated by the specialized vascular network, the choroid plexus. The ventricles are directly connected to the spinal canal (within the spinal cord).

growth factor - usually a protein or peptide that will bind a cell membrane receptor and then activates an intracellular signaling pathway. The function of the pathway will be to alter the cell directly or indirectly by changing gene expression. (eg VEGF, shh)

pericardium - covers the heart. Formed by 3 layers consisting of a fibrous pericardium and a double layered serous pericardium (parietal layer and visceral epicardium layer).

pericytes - (Rouget cells) cells located at the abluminal surface of microvessels close to endothelial cells, mainly found associated with CNS vessels and involved in vessel formation, remodeling and stabilization.

pharyngeal arches (=branchial arches, Gk. gill) series of cranial folds that form most structures of the head and neck. Six arches form but only 4 form any structures. Each arch has a pouch, membrane and groove.

truncus arteriosus - an embryological heart outflow structure, that forms in early cardiac development and will later divides into the pulmonary artery and aorta. Term is also used clinically to describe the malformation where only one artery arises from the heart and forms the aorta and pulmonary artery.

vascular endothelial growth factor - (VEGF) A secreted protein growth factor family, which stimulates the proliferation of vasular endotheial cells and therefore blood vessel growth. VEGF's have several roles in embryonic development. The VEGF family has 7 members (VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, VEGF-F, and PlGF) that have a common VEGF homology domain. PIGF is the placental growth factor. They act through 3 VEGF tyrosine kinase membrane receptors (VEGFR-1 to 3) with seven immunoglobulin-like domains in the extracellular domain, a single transmembrane region, and an intracellular tyrosine kinase sequence.

vasculogenesis - the formation of new blood vessels from mesoderm forming the endothelium. Compared to angiogenesis that is the process of blood vessel formation from pre-existing vessels.